Transmit beamforming is a technique for forming directional antenna patterns in a multi-antenna system. Transmit beamforming in a phased array transmitter is accomplished by introducing phase shifts in each transmitting branch such that the output signals produce a wavefront in a certain direction.
FIG. 1A depicts a known implementation of a phased array transmitter 100. The input IF-A signal is fed to a splitter 110-A which splits the signal into a plurality of input IF signals IF-1A, IF-2A, IF-NA. The input IF signals are passed on to a plurality of transmitting branches 120-1, 120-2, 120-N, each comprising a phase shifter 121-1A, 121-2A, 121-NA for phase shifting the IF signal and a mixer 123-1, 123-2, 123-N for up-converting the phase-shifted IF signal to radio frequency, RF. In each transmitting branch, the up-converted signal RF-1, RF-2, RF-N is passed through a band-pass filter 124-1, 124-2, 124-N before being emitted from an antenna element 130-1, 130-2, 130-N.
The above implementation has the advantage of relatively simple phase shifting at an intermediate frequency (or alternatively at baseband). Moreover, a multi-channel application requires that each transmitting branch has its own up-converting mixer. A multi-channel implementation of the same phased array transmitter is shown in FIG. 1B. The multi-channel phased array transmitter 100′ is identical to 100, with the addition of at least one other channel IF-B. The additional channel has a separate splitter 110-B and phase shifters 121-1B, 121-2B, 121-NB. A combiner 125-1, 125-2, 125-N combines the two phase shifted signals before the signal is up-converted to RF. The mixer 123-1, 123-2, 123-N mixes the LO and IF signals in order to provide an output RF signal.
The wanted RF output of the mixer may, for example, be RF=LO+IF, but the output will also comprise unwanted spurious signals at any frequency n*IF+m*LO where n and m are any integer not equal to one. A disadvantage of the phased array transmitters of FIGS. 1A and 1B is that the band-pass filters 124-1, 124-2, 124-N will not be able to suppress unwanted frequency components efficiently due to space constraints. Passive filtering is space consuming and becomes troublesome in high frequency applications where each channel has a specific area to use defined by the carrier frequency. For antenna arrays operating at high microwave frequencies (e.g. above 20 GHz) there is no possibility to have bulky filters behind each antenna element since these must be spaced no more than half a wavelength apart. Another problem is that for higher carrier frequencies the relative frequency difference between LO and RF frequencies decreases. This makes filter design even more troublesome when it comes to tolerances losses and even material variations. Hence, band-pass filtering alone will not be sufficient to achieve suppression of the unwanted spurious signals.
The capacity of a communication system using the phased array transmitters described above will be limited by the interference from the unwanted spurious signals, mainly LO and LO-IF. Hence, in order to increase capacity further the problem of interference from unwanted spurious signals needs to be solved. Currently there exist no efficient implementations that remedy this problem. Hence, there is a need for an improved phased array transmitter that provides suppression, or at least control, of the unwanted spurious signals.